To raise the purifying efficiency of a catalytic converter, high-temperature exhaust gas must be supplied to it. To make such supply possible, the catalytic converter should preferably be provided in the exhaust-gas passage near the engine proper.
At the same time, the exhaust-gas passage of each cylinder should preferably be formed as long as possible and separated from the exhaust-gas passages of the other cylinders, in order to prevent the pulsating exhaust pressure of each cylinder from interfering with the exhaust stroke of the other cylinders.